The present invention relates to a drill bit, in particular for percussion drilling of smaller holes, comprising a bit body having button inserts at its front end forming in an outer row of circumferentially spaced inserts, the inserts of said outer row being disposed at an acute angle with the longitudinal axis of the bit.
Drill bits equipped with button shaped inserts instead of chisel shaped inserts have shown to result in a more suitable attack against the bottom of the drill hole, primarily because of the many attack points against the hole bottom occurring as a result of using button inserts. As a result thereof the drill bit is given a more calm running through the drill hole than compared with chisel bits, which decreases the strains exerted on the drilling equipment. Simultaneously more fine-grained drill cuttings, easy to flush away, will be achieved as a result thereof.
Heretofore, however, it has been technically possible only to use button bits for the drilling of larger holes in hard rock formations. The use of small dimensioned drill bits with button inserts has been limited to relatively loose rock formations, such as coal, schist, limestone and similar. The technical problems connected with the manufacture of button bits suitable only for hard rock formations have increased with decreased dimension of the drill bit. The primary reason for this has been that the percussive forces applied to the button inserts from the rear percussive rod are too vigorous for small button inserts leading to fracture of the inserts during drilling in hard rock formations. Fracture of button inserts in larger dimensioned drill bits also seem to be a result thereof.
In accordance with the present invention it has surprisingly been possible to provide a drill bit with button inserts with which the disadvantages of the aforementioned kind can be overcome. The drill bit of this invention is provided with a conically shaped shaft portion to be connected to a correspondingly shaped percussive rod portion, and said drill bit body is provided with a button insert at the centre of the front end.
It has been found that a frictional joint of conical shape between the bit body and the drill rod is able to substantially dampen the percussive forces exerted to the bit body from the rod which positively affects the strength of the button inserts. By providing simultaneously a central button insert, which preferably extends axially in front of the other inserts, a more favorable directing of the drill bit and, as a result thereof, more straight drill holes will be achievable. The strains will also be more favorably distributed between the centrally and peripherally located button inserts.